A frequency converter is used for converting alternating current of fixed frequency, as it is drawn, for example, from a supply network, to an alternating current of variable frequency, for operating a motor.
A frequency converter has a rectifier circuit, a post-connected, intermediate circuit or link having an intermediate-circuit capacitor or link capacitor, and a post-connected inverter.
When switching on the frequency converter, the intermediate-circuit capacitor must first be charged. In this instance, very high currents may flow when insufficient measures are taken to limit these currents, for in the uncharged state, the intermediate-circuit capacitor acts as a short circuit.
Therefore, charging resistors are used which limit the charging current. These charging resistors may only be by-passed after the charging of the intermediate-circuit capacitor is at least substantially complete.
The charging current becomes smaller and smaller during the charging process, when the intermediate-circuit capacitor continuously charges.
In order to complete the charging of the intermediate-circuit capacitor as rapidly as possible, the charging resistors should be sized to be as small as possible. Therefore, it is possible for them to allow currents, which may produce a considerably high power loss in the resistors.
At the end of the charging process, it may be checked if the intermediate circuit or link has reached its required voltage. In this instance, the charging resistors are bypassed. Otherwise, the frequency converter is removed from the supply network or switched off.
If the intermediate circuit cannot be charged, since an intermediate-circuit short circuit or a high discharging current is present, this means that the charging resistors must withstand a considerably greater power loss, i.e., that the charging resistors burn out.